12 electric fans are pretty simple and easy.
You can use batteries (i.e. a charged car battery would last for about 24 hours per charge, or constantly charging it with a battery charger).
I like the previous cell phone charger idea. That would probably work well, but don't be surprised if your fans pull too much current and overheat the cell chargers.
An old laptop computer power chord (with a 12 V output) would be sweet to use, too. You could find all kinds of junk power cords lying around that put off 12 V. Like old answering machines, or cordless telephones bases, etc.
Then just twist the fan wires together with the cord and plug it in. As long as your house is up to code in the breaker box, you have no fear of exploding or burning anything down.